This application is related to U.S. patent application titled "Optimal Filling of a Polygon Facet by Beam Reprofiling" Ser. No. 07/990,492 (D/92542) filed concurrently herewith (common assignee).
This invention relates to an optical system which changes the intensity profile of a laser light beam from a non-uniform distribution to a uniform distribution and more specifically, to a raster scanner in which the intensity of a light source is reprofiled by a pair of binary diffraction optic lenses.
Binary diffraction optic lenses are made using the same techniques used to fabricate VLSI circuits, as disclosed in Binary Optics Technology: The Theory and Design of Multi-level Diffractive Optical Elements by G. J. Swanson of the Lincoln Laboratory at the Massachusetts Institute of Technology, (Technical Report 854, Aug. 14, 1989) and the resulting U.S. Pat. No. 4,895,790. A designer develops an idealized diffractive surface structure mathematically, then using a computer, defines a series of precise, microlithographic masks. A mask pattern is printed into a photoresist coating using a UV light source and then transferred into the optical substrate by ion milling or plasma etching.
Referring to FIG. 1, a conventional raster scanner system utilizes a light source 12, a modulator 14, a multi-faceted rotating polygon mirror 16 as the scanning element and a photosensitive medium 18. The light source 12, which is typically a laser source, emits a light beam 20 and sends it to the modulator 14. The modulator 14 receives pixel information and modulates the light beam 20 in accordance with the pixel information. The raster scanner system utilizes overfill scanning and therefore, the modulated light beam 20 will be directed onto two facets 22 of the rotating polygon mirror 16. The rotating polygon mirror 16 reflects the light beam 20 and also causes the reflected light 20 to revolve about an axis near the center of rotation of the rotating polygon 16 and scans a line. This reflected light beam 20 can be utilized to impinge upon a photographic film or a photosensitive medium 18, such as a xerographic drum at the output of the imaging system.
Referring to FIG. 2, typically, a laser light beam has a Gaussian distribution 30 which has a maximum intensity at the center C of the beam and a gradually decreasing intensity towards the outer shell S of the beam. In an overfilled raster scanner system in which a light beam is wide enough to cover at least two facets, an electro-optic crystal 14a used in the modulator 14 should be wide enough to encompass the width w of the light beam. However, since the intensity of the light beam varies from the center C of the beam to the outer shell S, the surface of the electro-optic crystal 14a of the modulator 14 does not receive a uniform intensity. The center portion Ca of the electro-optic crystal is exposed to the maximum intensity from the center C of the light beam and the rest of the electro-optic crystal is exposed to less intensity. For example in lithium niobate crystal the non-uniform light excites carriers from impurities such as iron in the crystal. These carriers move in the crystal by diffusion and under the influence of electric fields are displaced to new positions and re-trapped, which results in specially varying refractive index changes due to the electro-optic effect. This phenomenon of index variation is called the "photorefractive effect", or "optical damage" and is well documented in the literature. It is desirable in the case of information storage, but undesirable in the case of electro-optic modulator applications, since the index changes persist for long times and cause undesirable diffraction of light passing through the crystal. In particular the damage depends on the intensity of the light, which is not uniform in the case of a Gaussian beam.
Since the damage is greater near the center Ca of the electro-optic crystal, the performance of the center of the crystal degrades more than the performance of the rest of the crystal causing a non-uniform performance. As damage occurs optical absorption increases accelerating the damage, further exasperating the damage and a destructive cycle is set in motion. With a non-uniform performance, the performance of the center Ca of the crystal can drop to an unacceptable level while the performance of the rest of the crystal remains at the acceptable level. This effect is exaggerated when the intensity is sufficiently high that the outer areas can perform their proper function, resulting in excessive intensity near the crystal center leading to rapid doferoriaction. This problem can not be corrected by readjusting the modulator 14. Readjustment of the modulator 14 can correct the performance of the center of the electro-optic crystal however, it over corrects the performance of the rest of the electro-optic crystal. Therefore, an electro-optic crystal, which is damaged only at the center, is no longer useful and it has to be replaced by a new electro-optic crystal.
It is an object of this invention to change the incident light beam intensity profile from a Gaussian distribution to a uniform intensity to the electro-optic crystal and thereby produce a nearly uniform damage in the region of the crystal, which performs optical modulation. Due to the difficulty of getting uniformity at the edges of the beam, the edges of the beam are designed to be outside the electro-optically modulated region of the modulator. The uniform intensity can be applied to other optical applications such as multi-channel AO modulators, or indeed any optical system application, which requires a uniform optical beam.
It is a further object of this invention to change the intensity profile of a light beam from any intensity profile to any desired intensity profile.